true. In the case of proteins it requires the modification that the nitrogen, which in combustion is given off as the free element, is excreted from the body combined with a certain amount of carbon, hydrogen, and oxygen, chiefly as urea, CON2H4, a compound which can itself be further oxidized (e.g. by combustion) to carbon dioxide, water, and free nitrogen.

When burned in a furnace, fats and other organic food constituents produce heat, a part of which may be converted into work by such a device as a steam engine. A fat oxidized in the body also produces work and heat, the work-producing mechanism being the muscles.

We can easily measure the quantity of heat which any pure foodstuff or any mixed food is capable of producing when rapidly oxidized. This measurement can be made in exactly the same way as in the case of fuels, i.e. by burning a small weighed sample of the food or foodstuff in compressed oxygen in the bomb calorimeter (see Fig. 32, p. 55), and noting the quantity of heat set free. It is also possible to measure the quantity of heat set free by a man or an animal maintained on a certain diet. This is done by confining the man or animal to the chamber of an animal calorimeter an apparatus which measures the quantity of heat given off from his body.

66

Figure 39 is an interior view of a respiration " calorimeter designed by the late Professor W. O. Atwater of Wesleyan University, Middletown, Conn., and Professor E. B. Rosa of the same institution, for experiments upon man. This apparatus was so constructed that no heat could escape through the walls of the chamber. The heat given off by the occupant was absorbed by a current of water flowing through the pipes at the top of the chamber. The quantity of water flowing through these pipes was measured and also its temperature as it entered, and again

1 In the illustration these pipes are partly, but not entirely, concealed by metal "shields," which could be raised or lowered to regulate the rate at which the heat was taken up by the water.

as it left, the chamber. The product of the quantity of water flowing in a given time and its rise of temperature represented the number of Calories given off by the occupant of the chamber. The chamber had a tightly sealed window and a porthole or large pipe through which food and other materials could be passed in and out, the porthole being kept closed at one end whenever it was opened at the other. Provision was also made for continually renewing the supply of oxygen, and the apparatus owes its name of "respiration" calorimeter to the fact that it was so designed that not only the heat, but also the amounts of carbon dioxide and water produced, and (as later developed) the quantity of oxygen used by the subject of the experiment could be measured. Experiments with this apparatus were sometimes continued for periods of ten or twelve days.

This apparatus was greatly improved and eventually rebuilt by Dr. F. G. Benedict. Figure 40 gives a view of one form of apparatus now in use, showing the exterior of the respiration chamber and the apparatus used in measuring the heat given off and the carbon dioxide and water excreted by the occupant of the chamber. An observer is shown seated at the observer's table, a post which is manned night and day during the course of an experiment. This observer is making the temperature observations and so controlling the instrument that no heat can pass through the walls of the respiration chamber. In front of the observer is a hanging support for the galvanometer, an electrical instrument used in the temperature measurements. On the floor in front of the observer (at the right of the picture) is a rack or table holding the apparatus through which the circulating air is passed for purification and analysis before being returned to the respiration chamber. Behind the observer's platform (i.e. to the reader's left) near the floor is a large cylindrical vessel in which the water, which passes through the pipes in the chamber to absorb the heat, is collected and weighed. And on the extreme left of the picture a second experimenter is seen talking through a telephone to the man inside the chamber who is serving as the subject of the experiment.

When such measurements are made, it is found that the quantity of heat produced by the oxidation of a fat is exactly the same when this oxidation takes place slowly in the human body as when it takes place instantaneously in the bomb

FIG. 40. The Atwater-Rosa-Benedict Respiration Calorimeter as modified by Langworthy and Milner. Exterior View. From the Yearbook of the U. S. Department of Agriculture, 1910, by permission.

calorimeter. The same is true of a carbohydrate such as starch or sugar. The quantity of heat produced by the oxidation of a protein in the body is less than that produced by the combustion of the protein in the bomb; but the difference is just the quantity of heat yielded by the combustion of that quantity of urea and other nitrogenous end products which would be formed in the body from the given quantity of protein. In other words, if we were to allow the oxidation of the protein to go on in the body of the man in the respiration calorimeter and then burn in the bomb calorimeter the excreta of the man, the total heat obtained would be the same as if we had burned the protein directly in the bomb calorimeter.

Making allowance for the average quantities of each class of foodstuffs lost in digestion (i.e. excreted in the feces), it is found that a pound of carbohydrate in the food yields about 1815 Calories, and a pound of protein the same; but a pound of fat yields 4080 Calories.

One pound of fat is therefore equal in fuel value to 2 pounds of either protein or carbohydrate.

Stated according to the metric system of weights these fuel values are:

Carbohydrates and proteins

Fats

4

Calories per gram 9 Calories per gram

When muscular work is done, the heart beats more rapidly and the breathing becomes both faster and deeper. The result is a quickening of the oxidation processes in the body. A larger quantity of assimilated food material is oxidized in a given time, and of course a larger quantity of heat is produced. But the quantity of heat given off from the body does not now amount to 1815 Calories for each pound of protein and each pound of carbohydrate and 4080 Calories for each pound of fat oxidized, for a part of the "fuel value " of the food is converted into mechanical work.